Adds support for rbenv

Here is a recipe for installing rbenv (an alternative to rvm). It's a combination of a few different recipes that I came across. It also installs a lot of dependencies necessary for installing ruby. I tried to use `sunzi.install` to make it OS agnostic, but I wasn't sure of the syntax (since it is mentioned that it should be used in an `if` statement). Let me know what you think.